1. Field of the Invention
The invention relates to a method for computing robust and improved Signal-in-Space Accuracy (SISA) parameters in a Regional or Global Navigation Satellite System (GNSS).
2. Discussion of Background Information
Satellite systems for global navigation allow to accurately determining the position on Earth or in the air. GNSS, such as for example the actually constructed European satellite navigation system, better known under the name “Galileo,” comprise a plurality of satellites and a control system.
The accuracy of a GNSS depends on several parameters, one of which is the quality of the knowledge of the orbit of each satellite and the time error of the satellite clocks with respect to the system time. This quality is expressed by the Signal-in-Space Accuracy (SISA). While this description refers to the term SISA used in the frame of Galileo GNSS the invention is not limited to this system and the term is rather understood to be general. In particular the corresponding parameter for the NAVSTAR-GPS GNSS is referred to as User Range Error (URE). Further, additional background regarding SISA can be found in the article by Medel et al., “SISA Computation Algorithms and their applicability for Galileo Integrity,” Proceedings of the 15th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2002), and additional information regarding Galileo can be found in the article by Werner et al., “GALILEO: Integrity Performance Assessment Results And Recommendations,” ION GPS 2002: 15th International Technical Meeting of the Satellite Division of The Institute of Navigation; Portland, Oreg.; USA; 24-27 Sep. 2002, the disclosures of which are expressly incorporated by reference herein in their entireties.
SISA is the leading parameter describing the actual quality of the navigation signals of the GNSS and is broadcast regularly by the GNSS. A safe value as small as possible with a high confidence is essential for all navigation services as user trust this parameter in a way that navigation decisions—partly even for critical operations—are based on this. A high confidence must thus be ensured for this parameter.
The classical SISA computation developed in the frame of Galileo project consists of two steps: (1) determination of Signal-in-Space Error (SISE) samples; and (2) overbounding of the resulting distribution.
However, this computation method suffers of some drawbacks:                the determination of errors is very conservative;        due to the determination procedure there are singularities (bi-modality) in the underlying probability density which prevent proper overbounding;        overbounding results are highly sensitive to the applied configuration parameters which are selected either manually or by improper methods.        
It is very likely that due to the identified weaknesses it is not possible to reach the required confidence at the end.